i've fit a number of boilers (under supervision of my RGI guy),
It depends on the trainee, if the trainee is competant to do the work to a high standard, why would the 50 something RGI guy put his aging body through all that hard work when his trainee can do most of it, and the supervisor will just check it over?
Because thats still illegal.
http://www.gassaferegister.co.uk/pdf/Who can legally work on a gas appliance.pdf